Low pass filter and electronic device

ABSTRACT

The electronic device ( 100 ) of the invention comprises a semiconductor device ( 30 ) and a low-pass filter ( 20 ), which are present in a stacked configuration, and which together include a phase locked loop. The low-pass filter is preferably embodied by vertical trench capacitors, and preferably comprises a drift compensation part. The device ( 100 ) can be suitably provided in an open loop architecture. In a preferred embodiment, the low-pass filter comprises a large capacitor (C 2 ) and a small capacitor (C 1 ) connected in parallel, the large capacitor (C 2 ) being connected in series with a resistor (R 1 ).

The invention relates to a low-pass filter comprising a large and asmall capacitor which are connected in parallel, the large capacitorbeing connected in series with a resistor.

The invention also relates to an electronic device provided with aphase-locked loop function comprising a comparator, a low-pass filterand a voltage controlled oscillator, the comparator and the oscillatorbeing part of a single semiconductor device and the low-pass filterbeing embodied by a small and a large capacitor.

Such an electronic device is commercially available as a Bluetoothmodule provided by Philips Semiconductors as BGB101A. Such a Bluetoothmodule comprises all high-frequency elements needed for the operationaccording to the Bluetooth protocol, including a transceiver, a poweramplifier for amplifying the output signal to an antenna, a low-noiseamplifier for amplifying an input signal from the antenna as well asnecessary switches, matching functions and filters. The antenna may bepart of the module, but can be provided separately as well.

The low-pass filter in this module is embodied by discrete capacitorsand a discrete resistor. The small capacitor has a value of 2.2 nF andthe large capacitor has a value of 22 nF. The filter forms aphase-locked loop (PLL) function together with the comparator and theoscillator. In the device, the loop gets an input signal via thecomparator. This compares the input signal to a reference signal fromthe oscillator and sends its output to the low-pass filter. The outputof the low-pass filter is used both as an output signal and as an inputfor the oscillator. The output of the oscillator is the reference signalused by the comparator.

The low-pass filter has two functions in this topology: it is a filterrejecting all high-frequency components. Moreover, it will assure thatthe frequency of the oscillator is maintained, when the device is in thesending mode. Hereto, it is of importance for the capacitors to be ableto keep the specified frequency. For this purpose, capacitors thatfulfill the NP0-standard are used in the known module. This standardprescribes that the capacitance varies less than 30 ppm/° C. over atemperature range between −30 and +85° C.

It is a disadvantage of the current solution that these capacitors arerelatively large. It is desired to reduce the space of the module, sothat it can be included more easily in all kinds of hand-held apparatus.

It is therefore a first object of the invention to provide areduced-size low pass filter of the kind described in the openingparagraph.

It is a second object of the invention to provide an electronic devicewith the improved low-pass filter.

The first object is achieved in that the filter is embodied on the basisof a semiconductor substrate with a first surface, in which the smalland the large capacitor are provided as vertical trench capacitors, thetrenches extending to the first surface on which the resistor isprovided. Experiments have shown that the drift of the low-pass filteraccording to the invention is within the specifications, both at −30°C., at room temperature and at +85° C. This enables the use of thefilter in a so-called open loop architecture.

Vertical trench capacitors are known per se, for instance from Roozeboomet al., Int. J. Microcircuits and Electronic Packaging, 24 (2001),182-196. There is, however, no teaching or suggestion that capacitorswith different capacitances can be integrated in a single substratetogether with desired resistors. It is not known either that in such asingle substrate the leakage of the capacitors and the temperaturestability are according to specifications.

In a preferred embodiment the semiconductor substrate further comprisesa drift compensation part. Such a drift compensation part is suitablyembodied as an RC-filter that is coupled to ground. It is connected inparallel to the filter. This may be embodied in that there is a separateconnection—i.e. solder ball or bond wire—from the drift compensationpart to the semiconductor device. The RC-filter is chosen such as tohave the same period (time constant) as the drift which occurs in theopen loop architecture.

The combination of low-pass filter and drift compensation part takesaway the disadvantages of open loop modulation in that no more expensivediscrete capacitors are needed. It furthermore has the advantage of anopen-loop modulation that a very pure signal is provided without anyspurious components as necessary in a closed-loop architecture.

In a further embodiment one end of the filter is connected to ground.This is an embodiment of the open loop architecture. It is preferredthat both the drift compensation part and the filter are connected toground, such that there is one connection to ground from the low-passfilter.

It is preferred that the small and the large capacitor are separated bya high-ohmic substrate zone with a resistance of at least 0.5 kΩ/cm.Herewith the leakage current can be reduced as desired. Preferably thesubstrate zone has a resistance of about 1.0 kΩ/cm. The substrateresistance is preferably increased by implantation, more preferably byimplantation with Ar gas.

In another embodiment, the trench capacitors have a dielectriccomprising silicon nitride. Such a dielectric does not show anyhysteresis. It has been found preferable to use a stack of siliconoxide, silicon nitride and silicon oxide as the dielectric. Such stacksturn out to have excellent breakdown voltages.

In a further embodiment the resistor comprises a layer of polysilicon,in which layer the upper electrodes of the capacitors are defined aswell. This is advantageous in view of processing.

In an even further embodiment, the semiconductor substrate furthercomprises diodes. Such diodes can be suitably embodied as a pn diode, aZener diode, a back-to-back diode, a front-to-back diode or a floatingdiode. The diodes can very well be combined with the high-ohmicsubstrate in that only part of the substrate is made high-ohmic. This isalso preferable for the etching of the vertical trench capacitors. Theadvantages of the integration of the diodes are at least twofold. First,the open loop architecture can be embodied in that the low-pass filterand the drift compensation part are each connected to diodes. A largerpart of the phase locked looped function can thus be integrated in thesemiconductor substrate. This allows that the number of connectionsbetween the filter and the semiconductor device is reduced. Itfurthermore allows for the available space to be used more efficiently,and hence for miniaturisation of the electronic device to be possible. Asecond advantage is that the semiconductor device can thus be morespecifically designed for its functions of comparator and oscillator. Itis for instance possible to use other substrate materials in which thediodes do not fit well.

The second object of the invention is achieved in that the low-passfilter according to claim 1 is present, which filter is assembled to thesemiconductor device in a stacked die construction. Due to the stackeddie construction, the space for the discrete capacitors is no longerneeded. Surprisingly, it was found therewith that the presence of thefilter on top of the semiconductor device does not influence anyinductors implanted therein.

In a suitable embodiment the semiconductor device is provided with afirst and an opposed second side, on which first side the low-passfilter is present and on which second side the semiconductor device canbe coupled to a heat sink. The semiconductor device must dissipate moreheat than the low-pass filter, and should thus be provided with a heatsink. This can be realized in two different manners; first of all, inthat the semiconductor device is attached with its second side to a heatsink on a carrier, or on a leadframe. The filter is then provided as acomponent on the first side of the semiconductor device. The secondembodiment is that the device is provided with a leadframe to which thesemiconductor device is attached. The filter can then act as a supportfor the semiconductor device, and its surface can be provided withinterconnects that are used for rerouting.

In a preferred embodiment the low-pass filter has lateral dimensionswhich are at most equal to that of the semiconductor device. In thisembodiment, the low-pass filter will be a component on top of thesemiconductor device. The connections between the filter and thesemiconductor device can be realized both with wirebonding and withsolder or metal bumps. In case bumps are used, the semiconductor deviceis preferably provided with a structure known as bond pads on active.Such presence of bond pads on top of areas in which transistors aredefined requires a stabilisation of the interconnect structure of thesemiconductor device. For instance, the bond pads may be provided asconducting tracks on top of the passivation layer. The use of bond padson active allows the filter to be positioned on any location on thefirst side of the semiconductor device. It is preferred that the deviceis encapsulated after the filter has been assembled to the semiconductordevice.

These and other aspects of the filter and the electronic device of theinvention will be further explained with reference to the figures, inwhich:

FIG. 1 shows a diagrammatical cross-section of a first embodiment of thedevice;

FIG. 2 shows a diagrammatical cross-section of a second embodiment ofthe device;

FIG. 3 shows a diagrammatical top view of the second embodiment

FIG. 4 shows an electrical diagram corresponding to the invention

FIG. 5 shows a birds' eye perspective view of a prototype of the filterof the invention;

FIG. 6 shows a schematic drawing of an application of the presentdevice. The figures are not drawn to scale and like reference numbersrefer to like parts.

FIG. 1 shows a first embodiment of the electronic device 100 of theinvention. It comprises the low-pass filter 20 and the semiconductordevice 30. The semiconductor device comprises electrical functions notshown, which generally include a transceiver and an oscillator. However,the semiconductor device 30 can be limited to an oscillator and acomparator only. The semiconductor device 30 is provided on a heat sink13 with a layer of electrically conducting glue or solder. It iselectrically connected to the filter 20 via metal bumps 24. The filter20 herein also acts as a carrier and comprises any interconnects thatare necessary for rerouting purposes. Finally, it comprises the contactsto the leadframe 10 via solder bumps 27. The leadframe 10 shown here isa leadframe of the HVQFN-type (High Voltage Quad Flat Non-Leaded) havinga first side 11 to which the filter 20 and the semiconductor device 30are attached. It further has a second side 12, on which the contacts 16,17 and the heatsink 13 are exposed. The leadframe is half-etched fromthe second side 12, leading to spaces 18, which are filled withencapsulating, electrically insulating material and from part of theencapsulation 80.

FIGS. 2 and 3 show a second, preferred embodiment of the electronicdevice 100 of the invention. FIG. 2 shows a diagrammaticalcross-sectional view and FIG. 3 shows a top view. According to thisembodiment, the filter 20 is positioned on top of the semiconductordevice 30 and is attached thereto by a layer of non-conductive adhesive26. The semiconductor device is here too provided on a heatsink 13,which is part of a leadframe 10. In this embodiment, the heatsink 13also acts as a ground plane. The electrical connections between thefilter 20, the semiconductor device 30 and the leadframe 10 are realizedwith wirebonds 31-34. Wirebonds 31 connect the filter 20 to thesemiconductor device 30. Wirebonds 32 connect the semiconductor device30 to the contact 17 at the leadframe 10. These connections 31,32 carrysignals. The wirebonds 33,34 are connections to ground 16. Althoughwirebonds are preferable in view of the ease of application, aflip-chipped connection with metal/or solder balls, or a connectionwherein both wirebonding and flip chip are used, can be usedalternatively. The connection 33 of the filter 20 to ground isparticularly suitable for an open-loop architecture. It is understoodthat the specific lateral position of the filter on the semiconductordevice 30 is a matter of implementation.

FIG. 4 shows an electrical diagram that is used in the preferredembodiment. In this embodiment, the filter does not only comprise asmall capacitor C1 and a large capacitor C2, which is connected inseries with a resistor R1, but also a drift compensation part. Thisdrift compensation part comprises a capacitor C3 and a resistor R2. Thesmall capacitor C1 has a value of 2.2 nF and the large capacitor C2 hasa capacitance of 22 nF. The capacitor C3 has a value of 5.6 nF. Thesevalues are used for an oscillator that acts at the frequency of theBluetooth protocol, that is 2.4 GHz. It will be clear that othercapacitances can be chosen for other frequency bands. Furthermore, inthe present example the filter is designed to operate at a singlefrequency only. It can be extended, however, to include the elements formore frequency bands as well.

FIG. 4 furthermore shows the coupling of the drift compensation part andthe filter to the circuitry. At the one end, both the filter and thecompensation part are coupled to ground. This is embodied in the exampleshown in FIGS. 2 and 3 in a combined connection to ground. At the sametime, the drift compensation part and the filter are connected inparallel. Each of them is thus present between diodes. The circuit partis furthermore provided with resistors R3 and R3′. The diodes and theresistors can be integrated in the filter 20 if so desired.

Experiments have shown that the present filter has the desiredproperties to act as a low-pass filter. The experiments have been doneat different temperatures, at different frequencies and for hoppingmodes at different frequencies, and at different tuning voltages.

Frequency GHz temperature 2.402 2.441 2.478 DH1 (kHz) 25° C. −17 −16 −18DH5 (kHz) 25° C. −29 −26 −30If the frequency is set to hopping mode, an average drift of −15 kHz forDH1 and −26 kHz for DH5 is found.

Frequency GHz temperature 2.402 2.441 2.478 DH1 (kHz) 85° C. −10 −19 −19DH5 (kHz) 85° C. 26 −25 −32In the hopping mode an average drift of −12 kHz for DH1 and −25 kHz forDH5 is observed.

Frequency GHz temperature 2.402 2.441 2.478 DH1 (kHz) −30° C. 15 13 15DH5 (kHz) −30° C. 40 17 −28In the frequency hopping mode an average drift is found of 19 kHz forDH1 and 42 kHz for DH5. An optimal value can be achieved by tuning thetransceiver.

Further measurements were done in a static environment in order to checkthe level of the tuning voltage V_(tune). This was done in view of thesemiconductor device 30 comprising a coil, and the filter 20 beingpresent on top thereof. The coil is used in the voltage controlledoscillator.

At a tuning voltage level V_(tune) of 3 volts and of 2.7 volts thefollowing was observed:

Frequency GHz 2.402 2.441 2.478 Vtune (V) 1.4 1.64 1.94 DH5 (kHz) −25−20 −16As is apparent from this, the differences of the tuning voltage level donot affect the data found. It can thus be concluded that the tuningvoltage is not affected by the specific location of the filter 20 on topof the semiconductor device 20.

FIG. 5 shows a bird's eye perspective view of a prototype of the filteraccording to the invention. In fact, the prototype shown is an array oftrench capacitors. These capacitors are provided in a semiconductorsubstrate 1 of Si. The lower electrode 2 comprises an n⁺ doped zone inthe substrate 1. Thereon a dielectric 3, actually of silicon nitride, isprovided. The top electrode 4 is realized in polysilicon, which isn⁺-doped. On top of this a metal layer is provided in which the contactsfor the top electrode 5 and for the lower electrode 6 are defined. Thepores in the substrate 1 have a diameter in the order of 1 μm and adepth of about 20 μm. The capacitance of the capacitors is defined bythe number of pores. Between the capacitors the substrate is madehigh-ohmic. This provides a barrier between leakage between theindividual capacitors C1, C2, C3. If desired, resistors can be definedon the surface of the substrate, for instance in the same layer ofpolysilicon as the top electrode 4 is defined in. If desired, planarcapacitors can be defined on the surface of the substrate. If desired,inductors can be defined in the metal layer used for the contacts 5, 6.The pores can be grown by dry-etching or wet-chemical etching, such asknown from Roozeboom et al., Int. Journal of Microcircuits andElectronic Packaging, 24 (2001), 182-196.

FIG. 6 shows the architecture of an application of the electronic device100 of the invention. In fact, this application is a front-end radiomodule 200 suitable for processing signals according to the Bluetoothprotocol. The module 200 comprises the semiconductor device 30 and thefilter 20. The semiconductor device 30 contains in this case both thevoltage controlled oscillator and the transceiver and the power andlow-noise amplifier (shown as triangles in the figure). The transceiverpart comprises the functions of the regulator 211, the control logic212, the synthesizer 213, the DC extractor 214 and the demodulator 215.In addition to the semiconductor device 30 and the filter, the module200 comprises a supply decoupling 205, with the supply voltages andground as inputs. The module 200 further comprises a transmit path inwhich a balun and filter 203 are present; a receive path in which abalun and filter 204 are present; a switch 202 between the transmittingand the receiving paths, and a band-pass filter 201.

1. A low-pass filter comprising a large and a small capacitor which areconnected in parallel, the large capacitor being connected in serieswith a resistor, characterized in that the filter is embodied on thebasis of a semiconductor substrate with a first surface, in which thesmall and the large capacitor are each provided as a single verticaltrench capacitor, the trenches extending to the first surface on whichthe resistor is provided and further characterized in that thesemiconductor substrate further comprises a drift compensation part. 2.A low-pass filter comprising a large and a small capacitor which areconnected in parallel, the large capacitor being connected in serieswith a resistor, characterized in that the filter is embodied on thebasis of a semiconductor substrate with a first surface, in which thesmall and the large capacitor are each provided as a single verticaltrench capacitor, the trenches extending to the first surface on whichthe resistor is provided and further characterized in that the small andthe large capacitor are separated by a high-ohmic substrate zone with aresistance of at least 0.5 kΩ/cm wherein the high-ohmic substrate zoneis situation to mitigate leakage.
 3. A low-pass filter as claimed inclaim 1, characterized in that the trench capacitors have a dielectriccomprising silicon nitride.
 4. A low-pass filter as claimed in claim 1,characterized in that the resistor comprises a layer of polysilicon, inwhich layer the upper electrodes of the capacitors are defined as well.5. A low-pass filter as claimed in claim 1, characterized in that thesemiconductor substrate further comprises diodes.
 6. An electronicdevice, comprising: a phase locked loop function including a comparator,a low-pass filter and a voltage controlled oscillator, the comparatorand the oscillator being part of a single semiconductor device and thelow-pass filter including a large and a small capacitor that areconnected in parallel, the large capacitor being connected in serieswith a resistor, wherein the low-pass filter is embodied on the basis ofa semiconductor substrate with a first surface, in which the small andthe large capacitor are each provided as a single vertical trenchcapacitor, the trenches extending to the first surface on which theresistor is provided and the low-pass filter being assembled to thesemiconductor device in a stacked die construction, and wherein thesemiconductor device is provided with a first and an opposed secondside, at which first side the low-pass filter is present and at whichsecond side the semiconductor device can be coupled to a heat sink. 7.An electronic device comprising: a phase locked loop function includinga comparator, a low-pass filter and a voltage controlled oscillator, thecomparator and the oscillator being part of a single semiconductordevice and the low-pass filter including a large and a small capacitorthat are connected in parallel, the large capacitor being connected inseries with a resistor, wherein the low-pass filter is embodied on thebasis of a semiconductor substrate with a first surface, in which thesmall and the large capacitor are each provided as a single verticaltrench capacitor, the trenches extending to the first surface on whichthe resistor is provided and the low-pass filter being assembled to thesemiconductor device in a stacked die construction, and wherein thelow-pass filter has lateral dimensions which are at most equal to thoseof the semiconductor device.
 8. An electronic device comprising: a phaselocked loop function including a comparator, a low-pass filter and avoltage controlled oscillator, the comparator and the oscillator beingpart of a single semiconductor device and the low-pass filter includinga large and a small capacitor that are connected in parallel, the largecapacitor being connected in series with a resistor, wherein thelow-pass filter is embodied on the basis of a semiconductor substratewith a first surface, in which the small and the large capacitor areeach provided as a single vertical trench capacitor, the trenchesextending to the first surface on which the resistor is provided and thelow-pass filter being assembled to the semiconductor device in a stackeddie construction, and the phase locked loop being provided in an openloop architecture.
 9. A low-pass filter comprising a large and a smallcapacitor which are connected in parallel, the large capacitor beingconnected in series with a resistor, characterized in that the filter isembodied on the basis of a semiconductor substrate with a first surface,in which the small and the large capacitor are provided as verticaltrench capacitors, the trenches extending to the first surface on whichthe resistor is provided, and characterized in that the small capacitorand the large capacitor are separated by a high-ohmic substrate zonewith a resistance of at least 0.5 kΩ/cm.
 10. A low-pass filter asclaimed in claim 9, characterized in that the semiconductor substratefurther includes a drift compensation part.
 11. A low-pass filter asclaimed in claim 9, characterized in that one end of the filter isconnected to ground.
 12. A low-pass filter as claimed in claim 9,characterized in that the trench capacitors have a dielectric thatincludes silicon nitride.
 13. A low-pass filter as claimed in claim 9,characterized in that the resistor includes a layer of polysilicon, andupper electrodes of the large and small capacitors are defined in thelayer of polysilicon.
 14. A low-pass filter as claimed in claim 9,characterized in that the semiconductor substrate further includesdiodes.
 15. A low-pass filter as claimed in claim 9, wherein acapacitance of the large capacitor is approximately ten times largerthan a capacitance of the small capacitor.